Vortex flowmeters exploit a natural phenomenon known as vortex shedding. Vortices are shed from almost any non-streamlined body placed in a flow stream. An elongate body with a symmetrical cross section placed in a flow stream will shed vortices alternately from opposite sides. These alternating vortices then travel downstream in the flow, in what is known as the "Karman vortex street", in honor of the man who discovered this phenomenon. Over a fairly broad range of flowrates, the vortex shedding frequency is proportional to the velocity of the flow stream. Therefore, by applying an appropriate scaling factor to a measurement of the vortex shedding frequency, the velocity of the flow can be determined. By measuring the temperature and pressure of the fluid flow, and by applying suitable equations, such as Boyle's law for gases, the mass flowrate of the fluid flow can be determined.
In a vortex flowmeter, a bluff body is provided to generate a Karman vortex street and the vortex shedding frequency (and hence the velocity of the flow) is measured by a sensor. The sensor typically provides an output proportional to the deflection or strain generated in the sensor by the pressure fluctuations caused by the passing vortices.
Such deflective sensors have the disadvantage that they sense external vibrations in addition to the passing vortices. External vibrations degrade the output signal from the sensor, and restrict the lower end of the useable measurement range of the flowmeter. External vibrations are caused, for example, by the operation of nearby pumps, valves, or other process machinery.
Existing flowmeters have attempted to use sophisticated signal processing electronics to extract the vortex frequency signal from the overall sensor output signal. This approach is expensive, complex, and has only had limited success.
Other flowmeters have utilized a separate sensor to detect vibration. The output from this sensor is then subtracted from the overall signal to yield a better vortex frequency signal. While this approach has been more successful, the configuration of these flowmeters have disadvantages which will become apparent when compared with the vortex flowmeter of the invention.